2,293 research outputs found
Mars sample return mission: What level of complexity
The complexity of the U.S. Sample Return Mission is ultimately dependent on current mission funding and the projected direction of the U.S. space program. Despite these uncertainties, it is important to examine mission scenarios to address desired scientific objectives that can be summarized in the following list: (1) determine existence of climatic records in geologic records; (2) does Mars have a subpermafrost groundwater system; (3) fundamental questions on the existence of Mars biology; (4) what is the internal structure of Mars; (5) determine the systems for regolith formation; and (6) what is the contribution of meteorites to Martian geology and climate are presented. To address these objectives, the sample size, quantity and location must be established and whether this should be the only data searched for on the Martian surface. With this in mind, three mission scenarios are briefly discussed, in order of increasing complexity
The Centauri project: Manned interstellar travel
The development of antimatter engines for spacecraft propulsion will allow man to expand to the nearest stellar neighbors such as the Alpha Centuri system. Compared to chemically powered rockets like the Apollo mission class which would take 50,000 years to reach the Centauri system, antimatter propulsion would reduce one way trip time to 30 years or less. The challenges encountered by manned interstellar travel are formidable. The spacecraft must be a combination of sublight speed transportation system and a traveling microplanet serving an expanding population. As the population expands from the initial 100 people to approximately 300, the terraformed asteroid, enclosed by a man-made shell will allow for expansion over its surface in the fashion of a small terrestrial town. All aspects of human life - birth; death; physical, emotional, and educational needs; and government and law must be met by the structure, systems, and institutions on-board
Post-Impact Thermal Evolution of Porous Planetesimals
Impacts between planetesimals have largely been ruled out as a heat source in
the early Solar System, by calculations that show them to be an inefficient
heat source and unlikely to cause global heating. However, the long-term,
localized thermal effects of impacts on planetesimals have never been fully
quantified. Here, we simulate a range of impact scenarios between planetesimals
to determine the post-impact thermal histories of the parent bodies, and hence
the importance of impact heating in the thermal evolution of planetesimals. We
find on a local scale that heating material to petrologic type 6 is achievable
for a range of impact velocities and initial porosities, and impact melting is
possible in porous material at a velocity of > 4 km/s. Burial of heated
impactor material beneath the impact crater is common, insulating that material
and allowing the parent body to retain the heat for extended periods (~
millions of years). Cooling rates at 773 K are typically 1 - 1000 K/Ma,
matching a wide range of measurements of metallographic cooling rates from
chondritic materials. While the heating presented here is localized to the
impact site, multiple impacts over the lifetime of a parent body are likely to
have occurred. Moreover, as most meteorite samples are on the centimeter to
meter scale, the localized effects of impact heating cannot be ignored.Comment: 38 pages, 9 figures, Revised for Geochimica et Cosmochimica Acta
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Tracing the Ingredients for a Habitable Earth from Interstellar Space through Planet Formation
We use the C/N ratio as a monitor of the delivery of key ingredients of life
to nascent terrestrial worlds. Total elemental C and N contents, and their
ratio, are examined for the interstellar medium, comets, chondritic meteorites
and terrestrial planets; we include an updated estimate for the Bulk Silicate
Earth (C/N = 49.0 +/- 9.3). Using a kinetic model of disk chemistry, and the
sublimation/condensation temperatures of primitive molecules, we suggest that
organic ices and macro-molecular (refractory or carbonaceous dust) organic
material are the likely initial C and N carriers. Chemical reactions in the
disk can produce nebular C/N ratios of ~1-12, comparable to those of comets and
the low end estimated for planetesimals. An increase of the C/N ratio is traced
between volatile-rich pristine bodies and larger volatile-depleted objects
subjected to thermal/accretional metamorphism. The C/N ratios of the dominant
materials accreted to terrestrial planets should therefore be higher than those
seen in carbonaceous chondrites or comets. During planetary formation, we
explore scenarios leading to further volatile loss and associated C/N
variations owing to core formation and atmospheric escape. Key processes
include relative enrichment of nitrogen in the atmosphere and preferential
sequestration of carbon by the core. The high C/N BSE ratio therefore is best
satisfied by accretion of thermally processed objects followed by large-scale
atmospheric loss. These two effects must be more profound if volatile
sequestration in the core is effective. The stochastic nature of these
processes hints that the surface/atmospheric abundances of biosphere-essential
materials will likely be variable.Comment: Accepted by PNAS per
http://www.pnas.org/content/early/2015/07/01/1500954112.abstract?sid=9fd8abea-9d33-46d8-b755-217d10b1c24
Observational evidence of a slow downfall of star formation efficiency in massive galaxies during the last 10 Gyr
In this paper we study the causes of the reported mass-dependence of the
slope of SFR-M* relation, the so-called "Main Sequence" of star-forming
galaxies, and discuss its implication on the physical processes that shaped the
star formation history of massive galaxies over cosmic time. We use the CANDELS
near-IR imaging from the Hubble Space Telescope to perform the bulge-to-disk
decomposition of distant galaxies and measure for the first time the slope of
the SFR-Mdisk relation at z=1. We find that this relation follows very closely
the shape of the SFR-M* correlation, still with a pronounced flattening at the
high-mass end. This is clearly excluding, at least at z=1, the secular growth
of quiescent bulges in star-forming galaxies as the main driver for the change
of slope of the Main Sequence. Then, by stacking the Herschel data available in
the CANDELS field, we estimate the total gas mass and the star formation
efficiency at different positions on the SFR-M* relation. We find that the
relatively low SFRs observed in massive galaxies (M* > 5e10 Msun) are caused by
a decreased star formation efficiency, by up to a factor of 3 as compared to
lower stellar mass galaxies, and not by a reduced gas content. The trend at the
lowest masses is likely linked to the dominance of atomic over molecular gas.
We argue that this stellar-mass-dependent SFE can explain the varying slope of
the Main Sequence since z=1.5, hence over 70% of the Hubble time. The drop of
SFE occurs at lower masses in the local Universe (M* > 2e10 Msun) and is not
present at z=2. Altogether this provides evidence for a slow downfall of the
star formation efficiency in massive Main Sequence galaxies. The resulting loss
of star formation is found to be rising starting from z=2 to reach a level
comparable to the mass growth of the quiescent population by z=1. We finally
discuss the possible physical origin of this phenomenon.Comment: 21 pages, 15 figures, accepted for publication in A&
The selective effect of environment on the atomic and molecular gas-to-dust ratio of nearby galaxies in the Herschel Reference Survey
We combine dust, atomic (HI) and molecular (H) hydrogen mass
measurements for 176 galaxies in the Herschel Reference Survey to investigate
the effect of environment on the gas-to-dust mass ()
ratio of nearby galaxies. We find that, at fixed stellar mass, the average
ratio varies by no more than a factor of 2
when moving from field to cluster galaxies, with Virgo galaxies being slightly
more dust rich (per unit of gas) than isolated systems. Remarkably, once the
molecular and atomic hydrogen phases are investigated separately, we find that
\hi-deficient galaxies have at the same time lower
ratio but higher ratio than \hi-normal systems. In
other words, they are poorer in atomic but richer in molecular hydrogen if
normalized to their dust content. By comparing our findings with the
predictions of theoretical models, we show that the opposite behavior observed
in the and ratios is
fully consistent with outside-in stripping of the interstellar medium (ISM),
and is simply a consequence of the different distribution of dust, \hi\ and
H across the disk. Our results demonstrate that the small environmental
variations in the total ratio, as well as in the
gas-phase metallicity, do not automatically imply that environmental mechanisms
are not able to affect the dust and metal content of the ISM in galaxies.Comment: 11 pages, 6 figures, 2 tables. Accepted for publication in MNRA
Describing Signatures: a Key to Successful use of Remote Sensing for Forest Damage AssessmentDescrevendo Assinaturas: a Chave para o Uso Bem Sucedido do Sensoriamento Remoto para Avaliação dos Danos Florestais
Forest damage caused by a variety of agents: wildfire, climatic events, mammals, insects and diseases is often highly visible and can be assessed by remote sensing. Certain characteristics of the damage, as seen via remote sensing, can provide clues to the identity of the agent(s) responsible for the damage. These include the ability to recognize the host tree(s) affected, color and texture of the foliage of affected trees, distribution of damage, size of affected trees and portion of tree crown affected. Examples are provided from the United States and southern Brazil of how combinations of these characteristics are helpful in the identification of damaging forest agents during aerial sketchmap surveys, interpretation of large to medium scale aerial photos or airborne video imagery.Resumo Danos florestais causados por uma variedade de agentes: incêndios, eventos climáticos, mamíferos, insetos e doenças é frequentemente muito visível e pode ser avaliado por sensoriamento remoto. Certas características do dano, como vistos por sensoriamento remoto, podem proporcionar indícios para identificar o(s) agente(s) responsável pelo dano. Estas incluem a habilidade em reconhecer a(s) árvores hospedeiras afetadas, a cor e textura da folhagem da árvore atacada, a distribuição do dano, o tamanho das árvores afetadas e as partes da copa afetada. Exemplos são fornecidos dos Estados Unidos e do sul do Brasil de como as combinações dessas características são valiosas na identificação dos agentes causadores de danos durante os levantamentos aéreos expeditos, a interpretação de fotografias aéreas em escalas grandes a médias ou de imagens aéreas de vídeo
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